1. Field of the Invention
The present invention relates to an intake system for an internal combustion engine provided with an intake flow control valve in an intake port connected to a cylinder, and to a control method of the intake system. More specifically, the present invention relates to an intake system that prevents an intake flow control valve, which induces circular gas flow (vortex flow) in the cylinder according to the operating state of an internal combustion engine, from being rendered inoperable when the internal combustion engine is stopped.
2. Description of the Related Art
In order to stabilize combustion of a lean air-fuel mixture in a spark ignition internal combustion engine (hereinafter referred to as “engine”), it is important to produce gas flow such as tumble flow (vertical vortex) and swirl flow (horizontal vortex) in a cylinder, and it is necessary to enhance such gas flow across a wider operating range.
In the low-load operating range of the engine, where the throttle opening is small and the intake air amount is accordingly small, in particular, the fuel consumption and the emission tend to increase because air-fuel mixture is generally set to be slightly richer to stabilize combustion. In order to improve fuel economy and emissions, it is effective to induce circular flow of the intake air in the cylinder such as tumble flow and swirl flow to promote combustion with its strong turbulent flow.
The term “swirl flow” as used herein refers to flow of intake air circulating along the peripheral wall of the cylinder, which homogenizes intake air without producing turbulent flow, to promote combustion. Meanwhile, the term “tumble flow” as used herein refers to flow of intake air circulating along the axial direction of the cylinder, which improves combustion in the low-load operating range of the engine, because strong turbulent flow is produced as the tumble flow is deformed in the second half of the compression stroke.
In order to enhance gas flow (swirl flow and tumble flow) in the cylinder, conventional methods use an intake flow control valve to block part of the cross section of an intake port to cause intake air flowing in the intake port to flow towards one side of the intake port. In order to produce tumble flow, for example, the intake flow control valve is disposed on the lower side of the intake port to cause intake air to flow towards the upper side of the intake port, thereby enhancing tumble flow in the cylinder.
There is a clearance between the intake flow control valve and the housing that supports the intake flow control valve, and oil blown back from the combustion chamber occasionally adheres to the intake flow control valve through the clearance. A larger amount of oil adheres to the intake flow control valve when an engine key is turned off to stop the engine than during engine operation. When the engine is started, the engine speed is low and therefore the negative pressure acting on the intake flow control valve is small, which makes it difficult to blow away oil that has adhered around the intake flow control valve, resulting in a large amount oil remaining to adhere to the intake flow control valve. Oil adhering to the intake flow control valve increases the sliding resistance between the intake flow control valve and the housing, which changes the behavior of the intake flow control valve to occasionally cause a problem in the engine operation immediately after the engine is started. When the engine is stopped for an extended period, oil adhering to the intake flow control valve may harden and cause the intake flow control valve to become stuck to the housing, thereby preventing opening and closing operation of the intake flow control valve when the engine is started.
In view of the above problem, Japanese Patent Application Publication No. 9-203324 (JP-A-9-203324) describes an intake control device for an internal combustion engine that prevents an intake flow control valve from becoming stuck by reducing the amount of oil that adheres to the intake flow control valve irrespective of the operating state of the engine. The intake control device described in JP-A-9-203324 is provided with: a throttle valve provided in an intake passage for supplying intake air to each cylinder of an internal combustion engine to adjust the amount of air supplied to each cylinder; an intake flow control valve provided in the intake passage connected to each cylinder downstream of the throttle valve to adjust the intake period of each cylinder; and a control section for determining the open period and the close period of the intake flow control valve based on the operating state of the internal combustion engine. In the intake control device, the control section closes the intake flow control valve for a certain period from a predetermined timing at which a piston of each cylinder is moving when the internal combustion engine is stopped, and then keeps the intake flow control valve half-open.
The intake control device for an internal combustion engine reduces the contact area between the intake flow control valve and a housing for supporting the intake flow control valve to a minimum by keeping the intake flow control valve half-open after closing the intake flow control valve for a certain period when the engine is stopped. It is thus possible to prevent the intake flow control valve from becoming stuck to the housing by oil that hardens when the engine is stopped.
The intake flow control valve disclosed in JP-A-9-203324 has a rotary shaft and a valve element attached to the rotary shaft. The valve element is made up of a pair of left and right disks and a valve plate integrally formed between both the disks. A shaft support part is formed to extend coaxially with the rotary shaft on the outer side of the disk in the axial direction. One end of the valve element is supported for rotation by a bearing via the shaft support part, while the other end of the valve element is coupled to a motor. An attachment part for accommodating the valve element for rotation is in a cylindrical shape, and the valve element is assembled into the attachment part from its opening end. A slight clearance is kept between the outer periphery of the disks of the valve element and the inner peripheral surface of the attachment part when the valve element is rotated. When a predetermined time elapses after engine stop, the motor is de-energized with the intake flow control valve half-open. In this state, with almost no contact area between the intake flow control valve and the valve housing, the intake flow control valve is prevented from becoming stuck to the valve housing by oil that has adhered to the intake flow control valve. That is, it is possible to blow away oil that has adhered to the intake flow control valve and prevent the intake flow control valve from becoming stuck to the valve housing by once closing the intake flow control valve at the same time as the engine key is turned off and making the intake flow control valve half-open a few seconds later.
However, because the rotary shaft of the intake flow control valve disclosed in JP-A-9-203324 extends across the diameter of the intake pipe, the valve plate produces a large fluid resistance and thus results in a large pressure loss, even when the intake flow control valve is fully open. In order to avoid such a loss, for example, the cross section of the intake pipe is formed in a shape of a quadrilateral and an arc or an elliptical arc, the side surfaces of the valve element of the intake flow control valve are formed to coincide with the side surfaces of the quadrilateral of the intake pipe, and the rotary shaft of the valve element is provided along a plane perpendicular to the side surfaces of the valve element. The rotary shaft supports the intake flow control valve only on one side (on the base of the quadrilateral) (in so-called cantilever manner) so that the valve element extends along the bottom of the intake pipe when the intake flow control valve is fully open. In JP-A-9-203324, the intake flow control valve is made half-open with almost no contact area between the intake flow control valve and the valve housing to prevent the intake flow control valve from becoming stuck to the valve housing by oil that has adhered to the intake flow control valve. Meanwhile, in the cantilever intake flow control valve, the clearance in the fully close state is set to a minimum to prevent leakage, and is therefore the same as that in the half-open state. Therefore, it is not possible to prevent the intake flow control valve from becoming stuck, due to adhesion of oil or freezing of water, even with the intake flow control valve is half-open.